The present exemplary embodiments relate to a color toner composition, and to a limited coalescence suspension polymerization process for the preparation thereof. It find particular application in the production of a toner for use in developing an electrostatic image by electrophotographic, electrostatic recording and printing processes.
Present day toners are formulated from a range of potential components. Most toner compositions include at least a polymeric binder material and a colorant. Other commonly used components include black and colored magnetic oxides, charge control agents, internal additives to augment toner properties, such as aiding in deagglomeration and homogeneous distribution of the colorant in the toner composition, and external additives, to aid in the proper function of the toner. The components used in a particular toner formulation are dependent on the requirements of the machine in which the toner is ultimately intended to be used. For instance, the toner formulation must take into account such parameters as image quality, reliability, carrier life, toner shelf life, etc., all of which are intricately involved with the mechanical capability and design of the hardware of the machine.
Often, there is more than one component of a toner formulation which performs to eradicate certain undesirable properties of the toner. These same components may however, also contribute to other problems, or the combination of two or more components which affect the same toner properties may result in over-correction of a problematic area in the toner performance. Therefore, the combination of components selected to comprise a given toner composition must be carefully balanced, taking into account the full range of toner performance parameters which may be affected by each component and the interaction of each component with every other component of the toner composition, and the machine and its various components and systems.
Given that each of the foregoing parameters will affect toner performance in some manner, it is unlikely that any one toner will achieve optimum performance in all areas. Therefore, toner producers determine which parameters are most critical to the performance of a toner for a given purpose and which may be compromised, and to what extent.
Toner performance is determined by the combination of components, and by the physical, electrical and chemical properties of each. Such properties include pigment dispersion, particle size, particle size distribution, particle shape, bulk density, mechanical strength, flow properties, triboelectric charge, resistivity, softening point, blocking temperature, melt viscosity, and dispersion. Each of these parameters must be considered for each component in determining what components to combine and how to combine the components to achieve a balanced toner which produces an image having those properties determined to be most important for a specific toner. This choice of components is further influenced by economic and environmental concerns.
The bulk polymeric material of the toner generally functions as the binder for the colorants included in the toner formulation, but also affects many of the other toner functions, such as charging, electrical resistivity, and mechanical integrity, to name a few. Therefore, often times a combination of resins is used to achieve the desired performance. Polymers generally used in toner may be linear, branched or cross linked, and are chosen for their various properties and the manner in which these properties are likely to affect toner performance. For example, certain binder polymer properties affect the thermal performance of the toner. These properties include such binder parameters as glass transition temperature, melt viscosity, blocking temperature, and thermal integrity. In the same manner, the mechanical properties of the binder polymer, including such parameters as impact strength, adhesive/cohesive strength, and surface energy will also affect toner performance. Electrical traits such as triboelectric charge function, resistivity, and dielectric constant, and other miscellaneous features, such as moisture resistivity, % volatility, molecular weight, colorlessness, and pigment compatibility, all have an affect on the ultimate performance level of the toner in which the binder is used.
Among the most popular resins from which the toner resin may be selected are: acrylic resins, epoxy resins, polyamide resins, polyester resins, polyethylene resins, polystyrene resins, styrene-acrylic copolymer resins, and styrene-butadiene resins. As with all toner components, choice of resin is generally determined by the machine parameters and toner performance qualities sought.
Dispersed in the binder resin are the colorants used in the toner formulation. In monocomponent toners, magnetic oxide pigments are used for the purpose of enhancing the magnetic attraction between the toner and the developer roll assembly. Carbon black has historically been the most popular colorant used in black toners, as it strongly influences the triboelectric charging capability of the toner. However, more recent toners employ charge control agents to achieve and control this toner feature, thus allowing the use of more easily dispersed black colorants. The black colorant may also affect the flow characteristics of the toner and, therefore, is sometimes added in incremental amounts to the toner surface.
The charge control agents are also critical in full color printing. The equipment of today allows the reproduction of beautiful, photographic-quality full color images. The printer/copier machines generally employ one or more cartridges that dispense color toner, as well as black toner. The basic color toners used are magenta, cyan and yellow, though any number of other color toners are available. Generally, however, variations in color and tone or shade are produced by the combined printed affect of a basic color set of toners.
Most toner formulations also include any one or more of a number of materials known commonly in the industry as additives. These are generally fine particles that are physically blended with the toner. They may be attached to the toner by electrical means, mechanical means, or by mere physical mixing. These additives may be added to influence flow control, charge control, cleaning, fixing, offset prevention, transfer, conductivity control, humidity sensitivity control, and carrier life stability. Common additive materials include silica, metal oxides, metal stearates, fluoropolymer powders, fine polymer powders, rare earth oxides, waxes, conductive particulates, magnetite, carbon, and titanates. Choice of additives is critical, however, given that many of the additives affect more than a single toner property.
Clearly, given the vast number of components available in the industry for use in toner compositions, and given the propensity for many of the components to enhance some properties and at the same time to deleteriously affect others, choice of components is not a routine matter.
For example, it is known, as was set forth earlier, to produce toner compositions that include pigment colorants. Such compositions may use carbon black. Other color toners may use color pigments commercially available from a number of sources. It is critical to the quality of the printed image that the pigment or colorant used be homogeneously dispersed within the toner particles. This can be difficult to achieve given the propensity of pigment particulates to agglomerate, causing void areas in the toner particles that result in uneven color in the printed image. For this reason, many toner products include dyes instead of pigments. One problem with the use of dyes, however, is the lack of lightfastness and color density of the printed image. In an effort to overcome the problems of pigment dispersion in toner, the pigment has been used in the wet cake form. U.S. Pat. Nos. 5,667,929 and 5,591,552 disclose such a process for toner preparation. In these disclosures, pigment in the wetcake form was added to a mixture of linear polyester and toluene to form a pre-dispersion. The water was flushed, or displaced, by a resin/toluene solution, and then the toluene removed to generate a crushed powder of resin and pigment. While this method does increase pigment dispersion to some degree, printed images using the toner nonetheless exhibit very average print quality.
Therefore, one aspect of concern, and the one of most importance to this invention, is that of pigment dispersion. In an optimum toner, each toner particle will be consistent with respect to performance, and will exhibit a uniform distribution of colorant, charge control agent, additives, etc. The degree to which this uniform dispersion is achieved affects the resulting triboelectric charge, color, yield, and finally the printed image.
An additional concern in toner preparation is that of generating content-uniform particles exhibiting small particle size, at or below about 15 microns, and a narrow particle size distribution. Particle size, and the reduction thereof, is becoming increasingly more critical in toner production processes as newer generations of high resolution printing and copying equipment are developed.
Shape of the particle can be another concern. The more uniform the shape and the smaller the particle size, the better the printed image. Several patents that disclose a means of controlling the shape and particle size of the toner particles include U.S. Pat. Nos. 6,287,742, 6,461,783, 6,531,255 and 6,544,705. In the 6,287,742 patent, particulate resin, a dry pigment and optionally a charge control agent are combined, and this mixture is melted until the resin is in the molten state. The mixture is dispersed in an organic medium in which the resin is insoluble. A surfactant is also included in the organic medium. Under shear force and elevated temperatures, toner particles exhibiting consistent spherical shape and small size/size distribution are generated. In the U.S. Pat. No. 6,461,783 patent the resin polymer and dry pigment or dye colorant are combined with a vaporizable plasticizer which is then vaporized off after the mixture is subject to high shear mixing at elevated temperature. The vaporizing of the plasticizer introduces a surface roughness to the toner that aids in performance. While the forgoing achieve good results with respect to toner particle size/size distribution and shape, the disclosures nonetheless fail to address the problem of satisfactory pigment dispersion within the toner particles. The use in these disclosures of conventional or dry pigment, which remains difficult to uniformly disperse even with the processing shown in these patents, results in toners of lesser quality with respect to lightfastness and color density.
One means to achieve homogeneous or uniform pigment dispersion is set forth in U.S. Ser. No. 10/878,860, filed Jun. 28, 2004, to our common assignee, the disclosure of which is incorporated herein by reference. In that disclosure, the pigment colorant is added to the toner composition in the form of an aqueous liquid pigment concentrate dispersion.
Incorporation of waxes inside toner particles enables not only the anti-blocking properties but also the high-temperature anti-offset properties without applying any release agent such as silicon oil to fusing rollers (see, e.g., U.S. Pat. No. 6,458,502). Canon first disclosed wax containing chemical toners prepared by suspension polymerization and use them in single component laser printers. The wax forms separated domains and are encapsulated by the toner binder resin. This type of toners has become widely used in desktop laser printers without using fuser roll oil. Wax incorporation has been adapted by Avecia and DPI solutions for making emulsion aggregation toners and chemically milled toners respectively (for example DPI's U.S. patent application Ser. No. 10/366,369). Kodak disclosed evaporative limited coalescence processes for making toners with waxes (see U.S. Pat. Nos. 5,283,149 and 5,298,355). We have not been aware of any use of aqueous pigment dispersions in combination with aqueous wax dispersions disclosures on wax-containing toners produced by a suspension polymerization process; see for example U.S. Pat. No. 4,912,009. We hereby propose wax-containing toners prepared by a suspension polymerization process.
In addition, the current inventors have determined an alternative means by which to achieve uniform pigment dispersion within toner particles with volume average diameter of less than 15 μm. This is accomplished using an aqueous pigment pre-dispersion in combination with the processing steps set forth herein, to produce a toner suitable for generating a printed image with enhanced brightness of colors, visual density and vividness of color, each of which is a direct result of the quality of the pigment dispersion incorporated into the toner. Further, these toners exhibit narrow charge distribution and narrow particle size distribution.